As the aforementioned application describes, prior plastic films employed for packaging can have barrier properties improved by the application of a metallic material, such as a coating of aluminum, or by the application of a dielectric material, namely, an oxide of silicon, i.e. SiO or SiO.sub.2.
A variety of techniques have been employed for applying such barrier coatings, including thermal deposition, sputtering, plasma-enhanced chemical vapor deposition (PE-CVD), and electron-beam gun evaporation techniques (EBG).
With these earlier barrier layers, the barrier effectiveness appears to depend upon the deposition conditions, the adhesion of the film to the substrate and the compactness of the deposited film. Surprisingly, barrier performance is not significantly affected by a thickness of the deposited film. In other words, once a uniform and substantially continuous coating of the film is applied to establish the barrier, additional thicknesses or an increase in the thickness does not materially improve the barrier effectiveness.
With earlier systems for applying, especially dielectric films like SiO to a plastic film substrate, undesired coloring of the film resulted. More particularly, the application of SiO especially and to some extent SiO.sub.2 resulted in a yellowing of the plastic film as could be measured by absorption in the violet region of the visible light range on the coated film. The yellowing of the plastic film reduced the desirability thereof as a packaging film, in spite of the fact that the dielectric barrier coating did materially improve the resistance of the film to the passage of gasses such as oxygen and vapors such as water vapor therethrough.